There is a genuine demand for the high performance batteries with performance characteristics that include high power, high energy, high reliability and safety, longer life, as well as low cost and environmentally benign.
Various battery chemistries have been explored as higher energy density alternatives to conventional lead acid and nickel cadmium rechargeable batteries, as these incumbent battery technologies cannot keep up with the increasing energy requirements of new applications and also pose environmental issues with respect to production and disposal.
Zinc has long been recognized as the ideal electrode material, due to its high specific capacity (813 Ah/kg), low electrochemical potential (namely higher cell voltage), high coulombic efficiency, reversible electrochemical behavior, high rate capability, high abundance in the early crust and therefore low material cost, and environmental friendliness. Therefore, rechargeable zinc cells containing zinc electrodes, such as, for example, nickel/zinc, silver/zinc, MnO2/zinc and zinc air cells, are of significant interest. As compared to nickel cadmium cells, nickel/zinc cell has an open cell voltage over 1.72 V vs. 1.4 V for nickel cadmium cell. Significant environmental issues have been found in recent years with the manufacture and disposal of toxic nickel cadmium cells. Therefore, there is a strong need of developing high power, long cycle life and environmentally friendly rechargeable batteries with zinc as the anode material. Many batteries containing a zinc electrode are known and have been practiced in the art, including non-rechargeable zinc alkaline batteries.
Despite these advantages, conventional rechargeable zinc cells suffer short cycle life. This problem is caused by three major reasons: shape change of the electrode, dendrite shorting and electrode shedding during the cycle. In a conventional zinc electrode charge/discharge cycle, zinc is dissolved into an alkaline electrolyte during discharge and re-deposited onto the electrode during charge. Zinc tends to redistribute over a number of charge/discharge cycles, which causes a shape change of the electrode and reduces the battery capacity and cycle life.
The recent introduction of zinc ion rechargeable batteries in aqueous electrolytes has attracted great attention. Such zinc ion battery essentially consists of zinc metal as anode and ZnSO4 aqueous solution as electrolyte. In these batteries, α-MnO2 has attracted considerable attention as the zinc ion intercalation cathode due to its structure. Since α-MnO2 has a large open tunnel (2×2) structure, it was found that zinc ions can be intercalated and de-intercalated into the electrode material reversibly at high rate according to the following reaction:Zn2++2e−+α-MnO2→ZnMn2O4 
Theoretically, α-MnO2 has specific capacity of 616 mAh/g. In combination with the high specific capacity of Zn (i.e. 820 mAh/g), this new electrochemical coupling has a theoretical specific energy of 739 Wh/kg (assuming OCV˜2.1 V). However, when an aqueous electrolyte is used, the practical average cell voltage realized is only about 1.5 V and specific capacity of α-Mn2 is only 150 mAh/g,—equivalent to 190 Wh/kg on the cell level. However, the cycle life of these batteries fails to meet current application requirements due to the degradation of the zinc and α-MnO2 electrodes.
For large scale energy storage, system safety is of critical importance. Manganese oxide or its derivatives have been proposed as the cathode materials for lithium ion batteries, where non-aqueous electrolyte consisting of organic solvent and lithium salts is used. Due to the flammable nature of the electrolyte, the battery is not intrinsically safe.
A water-based electrolyte is an attractive alternative and alkaline electrolytes with pH greater than 7 have been explored to develop the battery system. The development of a rechargeable alkaline battery has attracted great attention in this aspect. In spite of early efforts, such as that disclosed in U.S. Pat. No. 4,929,520, rechargeable alkaline cells have suffered the severe drawback of very short cycle life (<50 cycles).
A significant breakthrough was made by Ford Motor Company in 1980s with the development of a MnO2 electrode that was truly rechargeable in the alkaline electrolyte. In U.S. Pat. Nos. 4,451,543 and 4,520,005, the chemically modified bimessite MnO2 electrode with introduction of foreign metal ions, including Ag, Al, Ba, Bi, Ca, Ce, Cu, K, La, Pb, Sb, Sn, Y and Zn ions. Bi doped bimessite MnO2 electrode maintained 85% theoretical capacity for over 800 cycles. An alternative method was also proposed by Kainthla et al. (R. C. Kainthla, D. J. Manko, U.S. Pat. No. 5,156,934 (1992)), which enabled the low of graphite addition in the electrode. Unfortunately, a product of zinc discharging is soluble in alkaline electrolyte in the form of zincate. The formation of zincate not only causes shape change and dendrite formation at zinc electrode but also affects the cycle performance of MnO2 electrode by reacting with manganese species to form electrochemically inactive species such as hetaerolite (ZnO—Mn2O3). In fact, once paired with zinc electrode in the full cell configuration, even chemically modified MnO2 electrodes cannot have a cycle life sufficient to make them commercially viable.
Another issue with the development of rechargeable alkaline Zn—MnO2 batteries is associated with the zinc electrode development. In an alkaline electrolyte, zinc has a tendency to form dendrites and to have shape change—which lead to shortened cycle life of the battery.
In order to address the issues associated with rechargeable Zn/MnO2 batteries with an alkaline electrolyte, neutral electrolytes such as ZnSO4 or ZnCl2 are employed. In a neutral electrolyte, zinc dendrite growth can be substantially hindered. However, zinc tends to passivate, forming inactive zinc hydroxide or zinc oxide. These materials cannot be effectively reduced during charging, leading to high polarization and cell capacity decay.
In addition, some phase types of MnO2 can also suffer capacity decay in neutral electrolyte. Spinel type material (LiMn2O4) has been evaluated as the cathode materials in the previous work; however, the material has rather low specific capacity of less than 150 mAh/g. The cycle life of the battery cannot meet the commercial requirements.
In U.S. Pat. No. 6,187,475, Oh et al introduce manganese salt in the electrolyte, and better cycle performance was achieved. However, commercially viable cycle life and zinc passivation issues were never resolved.
In WO2013112660 A1 and US 2015/0244031, Adamson et al disclosed an electrolyte formed with a divalent cation. However, Adamson did not mix the divalent cation with monovalent anions, and the cell performance was insufficient.
Recent uses of zinc ion rechargeable batteries in aqueous electrolyte solutions have attracted great attention. An exemplary zinc ion battery essentially includes zinc metal as anode, α-MnO2 as the zinc ion intercalation cathode and ZnSO4 aqueous solution as electrolyte. Since α-MnO2 has a large tunnel (2×2) structure, it was found that zinc ions can be intercalated and de-intercalated into the electrode material reversibly at high rate according to the following reaction:Zn2+2e−+α-MnO2→ZnMn2O4 
However, the actual specific capacity of the material is low and cycle life is limited.